530 



The following donations were then presented : — 



A collection of sixteen Original Views of the principal Buildings in 

 Dublin, by the late George Petrie, LL. D. : presented by the Marquis 

 of Kildare. 



A perfect and beautifully formed Celt, of micaceous grit, found in a 

 bog near Hacketstown, county of Carlow: presented by the Hon. and 

 Yen. Archdeacon Stopford. 



The Secretary read a letter from C. Darwin, Esq., returning thanks 

 for his election as Honorary Member of the Academy. 



MONDAY, MAY 14, 1866. 



Sir William II. W. "Wilde, M. D., Vice-President, in the Chair. 



John Barrington, Esq., D. L. ; John Casey, Esq. ; and William 

 Erazer, Esq., were elected Members of the Royal Irish Academy. 



The Rev. J. H. Jellett read a paper 



On a Eltjid possessing: Opposite Rotatory Powers for Rats at 

 Opposite Ends op the Spectrum. 



He commenced by alluding to the construction of an instrument which 

 he had exhibited and described to the Academy some time ago.* The 

 purpose of this instrument is to measure the rotatory power of a trans- 

 parent fluid, not directly, but by the method of compensation. Eor this 

 purpose the ray of polarized light, beforef its passage through the 

 fluid under examination, is transmitted through a fluid of an oppo- 

 site rotatory power ; and the construction of the instrument provides a 

 method of varying and measuring with exactness the length of the 

 column of this latter fluid through which the ray passes. The 

 fluids used for this purpose were, in general, the two well-known 

 species of oil of turpentine — namely, that which is obtained from 

 the Finns maritima of the South of France, and that which is obtained 

 from the Finns Austrcdis of North Carolina. The former of these, being 

 a left-handed substance, is used in the examination of fluids which, 

 like the solution of cane sugar, are right-handed ; and the latter, being 

 right-handed, for those substances which, like the solution of quinine, 

 and the great majority of fluids possessing the rotatory power,J are left- 

 handed. 



A means is thus obtained of comparing the rotatory powers of 

 all fluids for which the rotation is in the same direction ; but, in order 

 to compare those whose rotatory powers are opposite, it became neces- 

 sary, in the first place, to ascertain the relative rotatory powers of the 



* " Proceedings," vol. viii., p. 279. 



f The order of transmission is indifferent, but the construction of the instrument re- 

 quires that the light should passers* through the compensating fluid. 

 % So far at least as the author has examined them. 



